1. Field of the Invention
The present invention relates to a copying apparatus for reproducing an image by an electrographic process.
2. Description of the Related art
These kind of copying apparatuses are provided with a developing unit including a developing sleeve whose outer circumferential face allows the formation of a magnetic brush of developer consisting of magnetized carrier and toner; a toner supply roller which is adjacent to the developing sleeve and supplies a toner thereto; a power source for applying a developing bias voltage to the developing sleeve; and a power source for applying a toner-collecting bias voltage to the toner supply roller so as to develop an electrostatic latent image formed on an electrostatic latent image holding member by the magnetic brush formed on the developing sleeve.
Generally, a copying apparatus adjusts a reproduced image to one of several densities responsive to the tone of the image of an original document. In order to adjust the density of a reproduced image, the following methods are adopted: a developing bias voltage is changed; the gap between the developing sleeve and the photoreceptor drum is adjusted; or the height of a magnetic brush formed on the developing sleeve is regulated.
In the conventional copying apparatus, when the density of a reproduced image is adjusted by changing a developing bias voltage, the electric potential difference between the developing bias voltage applied to the developing sleeve and the toner-collecting bias voltage applied to the toner supply roller is changed, with the result that the amount of toner supplied from the toner supply roller to the developing sleeve is not balanced with the amount of toner collected from the developing sleeve to the toner supply roller, i.e., the density of the toner on the developing sleeve is changed. Thus, the density of the reproduced image is not as desired.
In order to overcome this problem, conventional copying apparatuses are provided with means for adjusting the gap between the developing sleeve and the toner supply roller or the height of developer in the form of a magnetic brush is regulated, but the construction is complicated, and further, it is difficult to control the means.